Cathode noble metals

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

Correct application of this principle can lead to what would appear to he peculiar recommendations. For example, if just one member of a couple is to be coated, it should be the noble member. Most coating systems leave holidays or tiny openings where the metal is exposed. If the active metal is coated, the area of exposure at the holidays can be quite small compared to the area of the noble metal, resulting in an unfavorable area ratio. On the other hand, if the noble metal is coated, the holidays provide a small cathodic area and hence a highly favorable area ratio with respect to the active metal. Similarly, if dissimilar metal fasteners must be used, they should be noble relative to the metals being fastened (see Case History 16.1). 

Note that zinc anodes are often used to protect steel and other relatively noble metals cathodically. In this case, the fasteners were acting as unintentional sacrificial anodes, protecting the stainless steel. Simple solutions to the problem would be to insulate the fasteners from the stainless steel electrically or to use stainless steel fasteners. 

Coatings of more noble metals than the substrate metal (e.g., Cu on Fe) are only protective when there are no pores. In other cases severe local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible. This protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. 

Coatings of less noble metals than the substrate metal (e.g., Zn on Fe) are only protective if the corrosion product of the metal coating restricts the corrosion process. At the same time, the formation of aeration cells is hindered by the metal coating. No corrosion occurs at defects. Additional cathodic protection to reduce the corrosion of the metal coating can be advantageous. Favorable polarization properties and low protection current requirements are possible but need to be tested in individual cases. The possibility of damage due to blistering and cathodic corrosion must be heeded. 

Noble metals should be specified for major structural units or components, particularly if the design requires that these are smaller than adjoining units. There is an unfavorable area effect of small anode and large cathode. Corrosion of a relatively small anodic area can be 100-1000 times more severe than the corrosion of bimetallic components, which have the same area submerged in a conductive medium. Hence, less noble (anodic) components should be made larger or thicker to allow for corrosion. In addition, provision should be made for easy replacement of the less noble components. 

Modem cells employ arrays of anodes (Ti02 coated with a noble metal) and cathodes (mild steel) spaced 3 mm apart and carrying current at 2700Am into brine (80-100gl ) at 60-80°C. Under these conditions current efficiency can reach 93% and 1 tonne of NaC103 can be obtained from 565 kg NaCl and 4535 kWh of electricity. The off-gas H2 is also collected. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

The addition of a small percentage of a noble metal to a base metal such as stainless steel or titanium can provide sites of low overvoltage for the cathodic reduction of dissolved oxygen or hydrogen ions. This permits larger currents and hence more positive potentials to be obtained at the anodic region, and promotes passivation under some circumstances . This effect has been demonstrated for stainless steels but has not been adopted in practice, since under other conditions the noble metal addition accelerates corrosion . 

The rate of corrosion and damage caused to the more negative metal will depend upon the relative sizes of the anodic (corroding metal) and cathodic areas. A small anode and a large cathode will result in intensive corrosion of the anodic area. On the other hand, if the anode is large compared with the cathode, the corrosion of the anodic area will be more general and less likely to result in rapid failure. For example, a steel rivet in a copper plate will be rapidly attacked in sea-water, whereas a copper rivet in a steel plate may lead only to slightly accelerated corrosion of the steel in the area adjacent to the rivet. Prediction of the rate of corrosion of the less noble metal... 

In principle, all metals may be protected by cathodic protection. In practice, it is not always relevant either because the metals are, to all intents and purposes, naturally immune to corrosion (the noble metals) and often not used as engineering materials or, being base metals, they are well protected by... 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

Introduction of electrOcheiiiicaUy active cathodes that facilitate passivation Raise potential by external e.m.f Additions of Pt. Pd and other noble metals to Ti, Cr and stainless steels Anodic protection of steel, stainless steel and Ti... 

In the galvanic detector, the electrochemical detector consists of a noble metal like silver (Ag) or platinum (Pt), and a base metal such as lead (Pb) or tin (Sn), which acts as anode. The well-defined galvanic detector is immersed in the electrolyte solution. Various electrolyte solutions can be used, but commonly they may be a buffered lead acetate, sodium acetate and acetic acid mixture. The chemical reaction in the cathode with electrons generated in the anode may generate a measurable electrical voltage, which is a detectable signal for measurements of DO. The lead is the anode in the electrolyte solution, which is oxidised. Therefore the probe life is dependent on the surface area of the anode. The series of chemical reactions occurring in the cathode and anode is ... 

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-eramres and potentials relevant to an operating PEFC, that the description of the cathode catalyst surface as Pt, implying Pt metal, is seriously flawed. Indeed, that a Reaction (1.4) acmally takes place at a Pt catalyst surface, exposes, Pt to be less noble than usually considered (although it remains a precious metal nevertheless. ..). Such a surface oxidation process, taking place on exposure to O2 and water and driven by electronically shorted ORR cathode site and metal anode site, is ordinarily associated with surface oxidation (and corrosion) of the less noble metals. 

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